Rapid and precise quality control inspections of the soldering and assembly of electronic devices have become priority items in the electronics manufacturing industry. The reduced size of components and solder connections, the resulting increased density of components on circuit boards and the advent of surface mount technology (SMT), which places solder connections underneath device packages where they are hidden from view, have made rapid and precise inspections of electronic devices and the electrical connections between devices very difficult to perform in a manufacturing environment.
Many existing inspection systems for electronic devices and connections make use of penetrating radiation to form images which exhibit features representative of the internal structure of the devices and connections. These systems often utilize conventional radiographic techniques wherein the penetrating radiation comprises X-rays. Medical X-ray pictures of various parts of the human body, e.g., the chest, arms, legs, spine, etc., are perhaps the most familiar examples of conventional radiographic images. The images or pictures formed represent the X-ray shadow cast by an object being inspected when it is illuminated by a beam of X-rays. The X-ray shadow is detected and recorded by an X-ray sensitive material such as film or other suitable means.
The appearance of the X-ray shadow or radiograph is determined not only by the internal structural characteristics of the object, but also by the direction from which the incident X-rays strike the object. Therefore, a complete interpretation and analysis of X-ray shadow images, whether performed visually by a person or numerically by a computer, often requires that certain assumptions be made regarding the characteristics of the object and its orientation with respect to the X-ray beam. For example, it is often necessary to make specific assumptions regarding the shape, internal structure, etc. of the object and the direction of the incident X-rays upon the object. Based on these assumptions, features of the X-ray image may be analyzed to determine the location, size, shape, etc., of the corresponding structural characteristic of the object, e.g., a defect in a solder connection, which produced the image feature. These assumptions often create ambiguities which degrade the reliability of the interpretation of the images and the decisions based upon the analysis of the X-ray shadow images. One of the primary ambiguities resulting from the use of such assumptions in the analysis of conventional radiographs is that small variations of a structural characteristic within an object, such as the shape, density and size of a defect within a solder connection, are often masked by the overshadowing mass of the solder connection itself as well as by neighboring solder connections, electronic devices, circuit boards and other objects. Since the overshadowing mass and neighboring objects are usually different for each solder joint, it is extremely cumbersome and often nearly impossible to make enough assumptions to precisely determine shapes, sizes and locations of solder defects within individual solder joints.
In an attempt to compensate for these shortcomings, some systems incorporate the capability of viewing the object from a plurality of angles. The additional views enable these systems to partially resolve the ambiguities present in the X-ray shadow projection images. However, utilization of multiple viewing angles necessitates a complicated mechanical handling system, often requiring as many as five independent, non-orthogonal axes of motion. This degree of mechanical complication leads to increased expense, increased size and weight, longer inspection times, reduced throughput, impaired positioning precision due to the mechanical complications, and calibration and computer control complications due to the non-orthogonality of the axes of motion.
Many of the problems associated with the conventional radiography techniques discussed above may be alleviated by producing cross-sectional images of the object being inspected. Tomographic techniques such as laminography and computed tomography (CT) are often used in medical applications to produce cross-sectional or body section images. In medical applications, these techniques have met with widespread success, largely because relatively low resolution on the order of one or two millimeters (0.04 to 0.08 inches) is satisfactory and because speed and throughput requirements are not as severe as the corresponding industrial requirements. However, no laminography inspection system has yet met with commercial success in an industrial application because of shortcomings in precision and/or speed of inspection. This is because existing laminography systems have been incapable of achieving the high positional accuracies and image resolutions necessary to solve industrial inspection problems while operating at the speeds necessary to make them practical in a production environment.
In the case of electronics inspection, and more particularly, for inspection of electrical connections such as solder joints, image resolution on the order of several micrometers, for example, 20 micrometers (0.0008 inches) is necessary. Furthermore, an industrial solder joint inspection system must generate multiple images per second in order to be practical for use on an industrial production line. Heretofore, laminography systems have not been able to achieve these speed and accuracy requirements necessary for electronics inspection.
Laminography systems for the production of cross-sectional images have taken several forms. One system is described in U.S. Pat. No. 3,928,769 entitled "LAMINOGRAPHIC INSTRUMENT." The radiation source and the detector described therein are mechanically coupled to achieve the required geometry and synchronized motion of the source and detector. This type of system has the disadvantage of having to move the relatively high mass of some combination of high mass elements including the radiation source, object under inspection and detector. This becomes especially difficult when X-ray tubes and camera equipment are to be used. The speed of this system is severely restricted due to the fact that it is extremely difficult to move these relatively large masses rapidly and precisely. This system also has limitations on the resolution that can be obtained due to the imprecision and degradation over time of the many complicated moving parts.
In another system described in U.S. Pat. No. 4,211,927 entitled "COMPUTERIZED TOMOGRAPHY SYSTEM," the mechanical motions of the radiation source and detector are electronically driven by separate stepper motors whose timing is controlled by the same computer. The motion of each component is referenced to a respective predetermined central calibration location. Thus, even though the source and detector are driven by the same computer, there is no direct link correlating the position of the source with the position of the detector. The performance of this system is also limited by the speed at which the massive radiation source and detector can be oscillated and by the precision, synchronization and stability of the moving parts.
In U.S. Pat. No. 4,516,252 entitled "DEVICE FOR IMAGING LAYERS OF A BODY," a plurality of radiation sources, each fixed in space at a different location, is used in lieu of a single oscillating source. The location of an image detector is moved electronically in synchronization with the activation of the plural sources. While this approach eliminates the problems inherent with mechanically moving the radiation source and detector, it entails the disadvantage in cost of requiring multiple radiation sources. The resulting image quality is also degraded because the desired blurring of out of focus features is not continuous, but rather discretized, due to the finite number of radiation source positions. Thus, unwanted features remain in the image as a plurality of distinct artifacts.
U.S. Pat. No. 2,667,585 entitled "DEVICE FOR PRODUCING SCREENING IMAGES OF BODY SECTIONS" shows a stationary X-ray tube with the radiation source motion provided by electrostatic deflection of the electron beam in the X-ray tube, thus causing the electron beam to trace a path over the surface area of a flat target anode. Opposite the X-ray tube is a detector image tube containing electron optics which deflect the resulting electron image onto a stationary detector. The deflection circuit of the X-ray tube and the deflection circuit of the image tube are driven from the same voltage supply so as to simultaneously drive the motion, of the X-ray source and the deflection of the resultant image in the detector. This system thus avoids many of the disadvantages associated with mechanically moving the radiation source and detector. However, this system has no provision for consistently maintaining the focus and energy of the electron beam as the beam is swept over the target surface. This causes the X-ray spot to vary in both size and intensity, which seriously limits the resolution achievable with the device. The use of electron optics to deflect the electron image also limits the detection resolution achievable with this device. This problem becomes especially severe as the image is deflected through large angles. Similarly, accuracy in the positioning of the X-ray spot is lost as the beam is deflected through severe angles. These characteristics substantially limit the resolution achievable with this technique. Furthermore, the technique is practical only for operation within a relatively small range of viewing angles, which limits the desired laminographic blurring effect of unwanted, features and consequently limits the resolution in a direction normal to the plane of focus.
All of the above described laminography systems are directed to performing body section radiography and, as such, are not designed to produce high resolution images in rapid succession. Furthermore, such systems need not operate in a continuous duty cycle nor in an environment compatible with the manufacturing of electronics.
Many of the deficiencies found in presently used electronic inspection systems could be overcome with a high resolution, high speed laminographic inspection system. Such a system would be particularly well suited for the inspection of electrical connections such as solder joints in electronic assemblies. A high resolution laminograph of a solder joint should be capable of unambiguously revealing features in the solder joint which are indicative of the joint quality. Unfortunately, even though many attempts have been made to utilize laminographic techniques in industrial inspection environments, prior systems have consistently fallen short of optimum performance because of poor image resolution or prohibitively long inspection times, or both. Techniques previously used to improve resolution invariably resulted in long inspection times. Likewise, techniques previously used to decrease inspection time have generally sacrificed image resolution. A need thus exists for a high speed, high resolution industrial laminography system capable of inspecting electronics in industrial environments.
A particular type of solder defect is the deposition of too much solder on a connection pad, known as an excess solder defect, and the deposition of too little solder on a connection pad, known as an insufficient solder defect. Excess solder may result in the presence of solder on portions of the circuit board, the component, or the conductive pad regions, where solder is unnecessary or even detrimental to performance of the connection. Insufficient solder may result in poor fillet quality and may not provide sufficient strength to the connection between conductive pads of a component and circuit board. Thus, it is desirable to provide a procedure for detecting excess/insufficient solder defects which utilizes the advantages provided by laminographic circuit board inspection techniques.
In the past, methods for detecting excess/insufficient solder defects have been unduly complex, and many of these methods have been based on unreliable assumptions which lead to misleading measurements. In addition to this, some systems require that multiple procedures for detecting excess/insufficient solder defects be used. For example, in one prior system, separate procedures are used to detect excess solder on the bottom lead of a through-hole joint, excess solder on the top lead of a through-hole joint, and excess solder on the contact pad of a surface mount solder joint.
One obstacle with detecting excess/insufficient solder defects is that, with prior circuit board inspection systems, it is difficult to obtain an accurate measurement of the solder thickness of some types of solder joints. Thus, some systems attempt to determine solder thickness by indirect means such as measuring the angle at which solder bulges from a solder joint. One prior device takes multiple X-ray images of the same solder joint from different levels and angles in order to obtain information relating to the characteristics of the solder joint. Unfortunately, this technique is time consuming due to the multiple images which must be processed for each inspected solder joint. Also, each view requires that many different procedures be used for determining the characteristics of the solder connection. Thus, there also exists a need for a high resolution circuit board inspection device which can simply and accurately determine the characteristics of solder joints on a circuit board in a short time period.